Nat. Biotechnol. 36, 31–39 (2018); published online 10 January 2018; corrected after print 13 March 2018

In the version of this article initially published, the acknowledgment “M.D. and J.M. were supported by the intramural research program of the National Library of Medicine, National Institutes of Health” was omitted. The error has been corrected in the HTML and PDF versions of the article.